The present invention relates to a data processing system having one or more disk drive devices, a cache store, at least one host processor, and at least one storage control unit for controlling transfers of data between the host processor, the cache store and the disk drive devices. More particularly, the present invention provides a method and means for overcoming faults in the system resulting from various conditions such as the inability to write a record on a disk after that record has been written-to or modified while in the cache store.
The above-referenced Application A discloses a cache/disk subsystem wherein a host processor wishing to write data on a disk issues a command to a storage control unit. The command specifies the operation to be performed (i.e. write) and the address of the disk space at which the operation is to take place. The storage control unit first checks to see if a copy of the data from the specified disk space is resident in the cache store. If it is, the data is then transferred from the host processor through the storage control unit to the cache store. If a copy of the data from the specified disk space is not resident in the cache store then the storage control unit causes the data from the desired disk space to be transferred to the cache store and overwritten with the data from the host processor.
Data is stored in the cache store by segments. The system uses the post-store method of destaging to the disk segments which have been written to while resident in the cache store. Written-to segments are destaged under the control of the storage control unit according to a variable priority scheme as disclosed in Application B. Thus, there may be an extended interval between the time a segment in the cache store is written-to and the time the storage control unit is ready to return the written-to segment to its disk space. During this interval the host processor may have discarded its normal directory linkages for the user program. Therefore, if a fault should occur such that the storage control unit cannot return a written-to segment to its disk space, the process for recovering from the fault could be very time consuming.
The present invention provides a file number which is issued by the host processor with a command, and stored under the control of the storage control unit for use if a fault occurs. Upon occurrence of a fault the file number may then be utilized by the host processor to speed up the error recovery.